Counting cracks in glass gives speed of projectile

A shattered windshield has a story to tell. The key tohearing it is counting the cracks.

The number of cracks that emerge in a plate of glass orPlexiglas relates to the speed of the object that broke it, researchersdemonstrate April 26 in Physical Review Letters. Thissimple relationship could prove useful for forensic scientists, archaeologistsand even astronomers.

Over the past century, most research into cracks hasfocused on parameters that determine whether a material remains intact whenstruck.

Nicolas Vandenberghe and his colleagues at Aix-MarseilleUniversity in France decided to try something different: They wanted to pushglass and other materials past their breaking points and study the resultingfractures. They wondered if they could connect the patterns of cracks to theproperties of the impact that created them, something no one had done before,Vandenberghe says.

So he and his team set up a shooting gallery. The targetswere small squares of glass and Plexiglas between 0.15 millimeters and 3millimeters thick. The researchers’ weapon was a gun filled with pressurizedair that fired 4-millimeter-wide steel pellets, about the size of BB’s, atspeeds ranging up to 432 kilometers per hour.

Knowing that cracks emerge within a matter of microsecondsof impact, Vandenberghe employed a high-speed camera that shoots 30,000 framesper second to capture the instant of collision. His team broke more than 100plates and then counted the cracks that extended outward in a star-shapedpattern from the point of impact.

The photographic evidence revealed a clear connection:After taking into account the type of material and its thickness, the number ofcracks doubled for every fourfold increase in the pellet’s speed. For example,a 70-kph pellet caused an average of four cracks in 1-millimeter-thickPlexiglas plates, while a 280-kph one made eight.

The study’s approach is clever, says Alan Zehnder, amechanical engineer at Cornell University. But he points out that most impactsdo not lead to such orderly, star-shaped crack patterns. He also notes thatVandenberghe’s materials were on a smaller scale than most everyday examples:The plates in the experiment are much thinner than a typical windowpane, hesays, and the steel pellets were significantly smaller than rocks that shatterwindshields.

Still, the study could yield useful understanding ofstar-shaped cracks. Vandenberghe wonders whether forensic scientists couldderive new information from broken windows at a crime scene. His work couldalso help archaeologists reconstruct cracked ceramic pottery or astronomersdecipher the collisions that produced fractures on the surface of a planet ormoon.

Video link:


N. Vandenberghe, R. Vermorel, E. Villermaux. Star shapedcrack pattern of broken windows. Physical Review Letters. April 26, 2013. doi:10.1103/PhysRevLett.110.174302 [Go to]

Suggested Reading

L. Grossman. Breaking it down. Science News. Vol. 177,February 13, 2010, p. 18. [Goto]

Web edition: May 1, 2013 link:
Print edition: June1, 2013; Vol.183 #11 (p. 15)

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